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MGB 1977 MGB will only run on full choke

Lavo

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Hi everyone, hope someone can help with this. A few weeks ago my car broke down on me causing me to be rescued off the motorway. After being towed home, I found that my spark plugs were black with soot i.e. Car had been running rich. I cleaned out the carbs, checking float levels and checking needles and seats were working in both carbs by blowing down the fuel tubes and making sure the needles cut off the air flow. I checked timing, replaced the distributor, checked valve lash and today I replaced the coil. A compression test shows 154, 154, 159, 154. I have managed to get the car running but as soon as I start to reduce the choke from maximum, the car stumbles and dies and also a sooty residue is being blown out of the exhaust. I have set the jets 2 full turns down from the bridge but nothing I have tried has worked. Does anyone have any ideas where I can go from here please.
Thanks.
 

JPSmit

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sounds very very rich!

1. what carb are you useing?

2. Do you have the air filter on (and if so, what happens if it is off?)
 
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Lavo

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Hi, I'm running HIF4s and the air filters are off whilst I'm working on it. I have put my hands over the intakes and the engine dies off.
 

Gliderman8

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I’m not familiar with HIF4 carbs, but I assume there’s a dashpot….
Have you checked to make there’s oil in there?
 

Mickey Richaud

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Sounds like it's time to start over...

2 turns down from level with the bridge for the jets is a starting point; you may need to raise them a flat or two. And I'd try lighter weight, single viscosity oil in colder weather. Maybe something like sewing machine oil.

Have a read here: https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF

And please keep us posted.

Mickey
 
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Lavo

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Cheers for that Mickey, I've printed the idiots guide off so I'll give that a go tomorrow with a lighter weight oil.
Dave.
 

Mickey Richaud

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You're most welcome! Hope it helps you in your quest.
 

JPSmit

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While you are at it, check your springs and cables. When it left you on the road the plugs were sooted (too rich) and when you got it reassembled it would only run on full choke (too rich) but when you let off the choke - at which point it should run better - even at half choke (less rich) - it dies, making me wonder if there is something mechanical between the cockpit and the carb that is binding/ kinking/ not fully opening or closing - could it be something as inane as the idle set too low?
 

DrEntropy

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HIF4's are a different animal but the principles are the same. Jet levels and balancing in particular.

And Mick hit on another possible with throttle return spring strength.

Mr. Adler mentions ColourTune and air flow devices, the ColourTune tool is a good investment, I've two so as to see both carbs at the same time. As for the AFM, the one illustrated in the PDF is now "counterfeited" and not near as accurate or reliable as the original ones.

This type has been more consistent recently, prices are all over the map tho:

AFM.jpg
 

Bob Claffie

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Only running with full choke and stalling out when it is eased off suggests a massive air leak somewhere in the intake system
 
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Lavo

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Thanks for that Bob, I have looked for a acuim leak bit can't find one. All pipes and connections have been sprayed with carb cleaner with no change whatsoever er in engine note. I'm at a loss.
 

JPSmit

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Lavo, what was happening when the car broke down initially? had you repaired/replaced anything recently?

Cars don't typically jump out of tune.

maybe we need to go further back.
 
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Lavo

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hi John, I havent replaced anything recently, ie in the past 12 months, the car was going really well up until this happened. Right before the car finally stopped, there was a loss of engine power, pressing the accelerator had no real effect, the engine began misfiring and I had to coast off the motorway.
 

DrEntropy

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That last description sounds like classic fuel pump failure!
 
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Lavo

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I have considered that but it does seem to be working ok. I've removed the fuel line from the carb and pumped fuel into a jar and there does seem to be a good supply.
 

JPSmit

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do you have a bright spark?
 
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Lavo

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I put a spark tester on each plug in turn yesterday and spark seemed good on each. I'm going to start again today. I'll check everything that's been pointed out to me and get back to the forum. Thanks to everyone who's commented.
 
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Lavo

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Today, I went back to the start,static timing to 10btdc valve lash all checked, checked points And plugs all ok. Turned jets 2 full turns rich from the bridge. Car started really easily on full choke. Broke out the carb cleaner to check for vacuum leaks. Still could find none. Once the engine was warm pressed choke into halfway, the revs went up momentarily before the engine stumbled and died. Still sooty exhaust. I'm now thinking a full carb rebuild but I don't really know if this will cure it.
 
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